Multiple myeloma (MM) is a tumor of terminally-differentiated plasma cells. During development, genetic abnormalities in the forming plasma cells can result in malignant MM cells, which travel through the bloodstream and deposit in bone marrow and other organs, causing the variable symptoms of MM. Initial transformative are thought to occur post-germinal, as suggested by the hypermutation of IGV genes. Collections of abnormal cells accumulate in bones, where they cause bone lesions and elevated calcium levels from myeloma cell release of IL-6, and in the bone marrow where they interfere with the production of normal blood cells, resulting in anemia and impaired immune response. Most cases of myeloma also feature the production of a paraprotein, an abnormal antibody that can cause kidney problems and interferes with the production of normal antibodies leading to immunodeficiency. The disease develops in 1-4 per 100,000 people per year. It is more common in men, and is twice as common in African American populations than it is in Caucasians. With conventional treatment, the prognosis is 3-4 years, which may be extended to 5-7 years with advanced treatments. Multiple myeloma is the second most common hematological malignancy (13%) and constitutes 1% of all cancers.
MM is diagnosed with blood tests, such as electrophoresis, peripheral blood smear, and microscopic examination of the bone marrow. Standard treatments include lenalidomide, bortezomit, thalidomite, doxirubicin, melphalan, cyclosphamide, prednisone, or dexamethasone. The disease is thought to progress due to dysregulation of the apoptotic mechanisms in plasma cells, which also is likely responsible for the resultant chemoresistance.
MM still remains an incurable disease despite improved treatment regimens that include bortezomib, lenalidomide and thalidomide. Other treatments that are thought to cause remission include steroids, chemotherapy, and stem cell transplants. Drug resistance, including resistance to topoisomerase II (topo II) inhibitors, is a major obstacle in the treatment of multiple myeloma. Cell adhesion-mediated drug resistance and stromal cell adherence are important parameters in the local bone marrow environment in patients with multiple myeloma and appear to be major determinants of drug resistance (Hazlehurst, et al., Reduction in drug-induced DNA double-strand breaks associated with beta1 integrin-mediated adhesion correlates with drug resistance in U937 cells. Blood 2001; 98:1897-903; Hazlehurst L A, et al., Cell adhesion to fibronectin (CAM-DR) influences acquired mitoxantrone resistance in U937 cells. Cancer Res 2006; 66:2338-45). Additionally, human multiple myeloma cell density is a determinant of sensitivity to topo II inhibitors (Valkov, et al., Cell density-dependent VP-16 sensitivity of leukaemic cells is accompanied by the translocation of topoisomerase IIalpha from the nucleus to the cytoplasm. Br J Haematol 2000; 108:331-45; Turner J G, et al., Human topoisomerase IIalpha nuclear export is mediated by two CRM-1-dependent nuclear export signals. J Cell Sci 2004; 117:3061-71). At increased cell densities, a considerable fraction of nuclear topo IIα(>90%) is exported to the cytoplasm, resulting in chemoresistance to VP-16 and doxorubicin. This appears to occur both in human myeloma cell lines and in CD138+ cells isolated from patients with multiple myeloma. Intracellular mislocalization of tumor suppressor or nuclear drug target has been shown to decrease the effectiveness of antineoplastic agents, such as with the tumor suppressors and chemotherapeutic targets p53, APC/β-catenin, FOXO, p21CIP1, p27KIP1, and topoisomerases I and II (Turner & Sullivan, CRM1-mediated nuclear export of proteins and drug resistance in cancer. Curr Med Chem 2008; 15: 2648-55).
For proteins greater than 60 kDa to be exported to the cytoplasm, they must be transported through the nuclear-pore complex (Cook, et al., Structural biology of nucleocytoplasmic transport. Annu Rev Biochem 2007; 76: 647-71). This transport mechanism involves the binding of chromosome-maintenance protein-1 (CRM1) to a leucine-rich nuclear export signal (NES) on the target protein. This complex is then transported through the nuclear pore into the cytoplasm (Dong, et al., Structural basis for leucine-rich nuclear export signal recognition by CRM1. Nature 2009; 458: 1136-41).
In multiple myeloma (MM), de novo drug resistance to topoisomerase (topo) II poisons occurs at high cell densities due to trafficking of topo IIα from the nucleus to the cytoplasm where it is no longer in contact with the DNA and thus unable to induce cell death (Turner, et al., Human topoisomerase IIalpha nuclear export is mediated by two CRM-1-dependent nuclear export signals. J Cell Sci 2004; 117: 3061-71; Turner, et al. Human multiple myeloma cells are sensitized to topoisomerase II inhibitors by CRM1 inhibition. Cancer Res 2009; 69: 6899-905; Valkov, et al., Cell density-dependent VP-16 sensitivity of leukaemic cells is accompanied by the translocation of topoisomerase IIalpha from the nucleus to the cytoplasm. Br J Haematol 2000; 108: 331-45; Engel, et al., The cytoplasmic trafficking of DNA topoisomerase IIalpha correlates with etoposide resistance in human myeloma cells. Exp Cell Res 2004; 295: 421-31). Topo IIα was previously demonstrated to be exported from the nucleus of human myeloma cells by a CRM1-dependent mechanism (Engel, et al., The cytoplasmic trafficking of DNA topoisomerase IIalpha correlates with etoposide resistance in human myeloma cells. Exp Cell Res 2004; 295: 421-31), and the NES for topo IIα was located to amino acids 1017-28 (site A) and 1054-66 (site B) (Turner, et al., Human topoisomerase IIalpha nuclear export is mediated by two CRM-1-dependent nuclear export signals. J Cell Sci 2004; 117: 3061-71). Blocking nuclear export with a CRM1 inhibitor or by siRNA has been shown to sensitize drug-resistant MM cells to topo II poisons (Turner, et al., Human multiple myeloma cells are sensitized to topoisomerase II inhibitors by CRM1 inhibition. Cancer Res 2009; 69: 6899-905).
However, use of CRM1 inhibition in cancer therapy has met with limited success. The first CRM1 inhibitor, leptomycin B, was found to efficiently inhibit nuclear export, but showed acute relative toxicities both in a human phase I trial (Newlands, et al., Phase I trial of elactocin. Br J Cancer 1996; 74:648-9) and in vitro. Leptomycin B in vitro studies found acute toxicity at concentrations <5 nmol/L for 1 hour. As such, new therapeutic targets are needed to further improve treatment outcomes of multiple myeolma.